In recent years, evidence has accumulated on the efficacy of transcranial electrical stimulation, using both direct current (transcranial direct current stimulation, tDCS), alternating current (transcranial alternating current stimulation, tACS), and random current (transcranial random noise stimulation, tRNS). Direct current stimulation has the ability to selectively sensitize or desensitize a particular brain area. Alternating current has the ability to entrain brain oscillations strengthening the EEG signal spectrum in a certain frequency band. Stimulation using random noise current induces consistent excitability in the target brain region.
Direct current stimulation applied during sleep has been shown to facilitate memory consolidation.
40 Hz alternating current stimulation applied during REM sleep has been shown to induce a state of consciousness known as “Lucid Dreaming”, in which the person becomes aware that he is dreaming while he is dreaming. Lucid dreaming has potential applications ranging from entertainment to treatment of PTSD and nightmares, and enhancement of athletic performance.
Many more applications of electrical brain stimulation during sleep are likely to emerge, such as reducing susceptibility to noise and possibly modulating sleep phases.
Currently there are no reports of adverse side effects from tDCS, tACS and tRNS, aside from mild itching and redness on the skin underneath the stimulation electrodes. The reason is that unlike electroconvulsive therapy, the currents used in modern brain stimulation techniques are extremely small. The stimulation is not meant to force neurons to fire in a specific pattern, but only to increase their natural likelihood to do so. The brain can be viewed as a multi-stable dynamic system which is sensitive to outside “nudges”. For this reason even a small current can have an impact on the overall functioning of the brain.
Unfortunately, attempting to affect the functioning of specific areas of the brain with electrical stimulation during sleep is currently a difficult undertaking, requiring medical expertise, skilled electrode positioning and application, and the involvement of a doctor or researcher throughout the stimulation.
In transcranial electrical stimulation research, it is common to monitor the EEG signal of a patient before and after stimulation, to verify whether the stimulation has had effects on the EEG spectrum. For example, alternating current stimulation can be used to potentiate frequencies around 40 Hz, and this effect can be verified by comparing the intensity of the patient's endogenous 40 Hz EEG waves before and after stimulation.
Transcranial electrical stimulation researchers normally utilize a clinical EEG monitoring device and a separate transcranial stimulation device. Electrodes are carefully applied in predetermined positions on the subject's scalp by medical personnel. Particular care is taken to ensure low impedance of the electrodes, particularly the stimulation electrodes, so as to reduce itching and redness.
A few costly devices are now available on the medical device market that allow both EEG monitoring and stimulation. The StarStim™ by Neuroelectrics is an EEG cap with a multitude of holes onto which a large variety of types of electrodes can be mounted (for instance, Ag—AgCl EEG electrodes, or sponge-type stimulation electrodes requiring periodic application of saline solution). Mounting the correct electrode type at the correct location is the responsibility of the doctor or researcher. Further, each electrode can be electrically configured to capture the EEG signal or apply a stimulation current. The configuration is controlled by medical personnel using a computer interface. This EEG cap is not suitable for, nor intended for, use during sleep. The battery is placed on the back of the head, thereby limiting the patient's sleeping position, and—for safety reasons—the product has been engineered to have an automatic shutdown time of 1 hour, thereby precluding its use throughout a full night's sleep.
In recent years, consumer devices have emerged which allow a user to monitor his/her EEG without the supervision of a medical practitioner. Such devices typically include a headband worn around the user's head, several EEG electrodes, and a small EEG monitoring device which is either embedded in the headband or structurally and electrically connected to the headband by means of snap fasteners, such as snap connectors.
The Zeo™ headband (by Zeo, Inc.) now out of production allowed monitoring of EEG signal bands during sleep to perform sleep staging. Many other commercial EEG headbands now exist on the consumer market (such as the Muse™ by Interaxon, or the Melon™ headband), though they are generally intended for wake-time EEG monitoring.
All these consumer devices do not include circuitry for brain stimulation, and even if they did the electrodes would be incapable of safely applying electrical current stimulation to the brain.
The Foc.us™ headset is at the time of writing the only commercially available tDCS headset which can be used by a user to self deliver transcranial direct current stimulation. However it is sold for the purpose of day time stimulation. Even if it was worn during sleep, it would be of no value because it would fall off.